Glacier Hydrology References Revision

Stuart et al., 2003

• Ground-penetrating radar profiles were used to collect data on an englacial channel system for cold-glacier austere Brøggerbreen, Svalbard

• The size of conduits varied from a large semicircular channel at 5m wide to a vertically elongated 2.5m channel close to the outlet as well as variations in the depth of water in the channel from 14 to 90% of channel height 

• Results of dye-tracing experiments combined with the low channel gradient of less than 2 degrees suggests that there is significant ponding along the channel; radar reflections were useful in determining the depth, dimensions and water content of these englacial watercourses

• This channel geometry was probably formed during a period of increased crevassing from the Little Ice Age, these channel dimensions were sourced from direct descent into the moulin to verify these interpretations through glacio-speleology

Samimi & Marshall, 2017

• Meltwater refreezing and storage in the superglacial snowpack can reduce and delay meltwater runoff from glaciers, but the effect of these processes are often uncertain for temperate alpine glaciers

• The temperature and meltwater content in the upper 50cm of the supraglacial snowpack of Haig Glacier was measured using thermistorics and Time Domain Reflectometry (TDR) probes and supplemented by automatic weather station data 

• These methods found strong diurnal cycles in snow water content through the summer melt season with subsurface refreezing only significant in May, after this overnight freezing was restricted to only a thin surface layer of the snowpack; here diurnal variation in supraglacial hydrology were revealed showing high levels of water stored in the daytime contrasting with minimum levels overnight 

• Overnight decreases in water content after May were associated with meltwater percolation and drainage; overall there was a negligible meltwater retention in the snow on a daily basis

Hock & Hooke, 1993

• During the 1989 melt season, 10 tracer experiments were conducted to investigate the seasonal, diurnal and spatial variations in the englacial drainage system of Storglaciarien, Sweden where dye was injected into moulins and its concentration and discharge monitored at the glacier terminus 

• Transit velocities ranged from 0.07 to 0.29 m/s, implying that drainage was initially taking place through a well-defined conduit system as part of a multi-branched aborescent network of wide, low passages; this dispersivity declined during the early part of the meltseason to reflect structural changes in conduits including a decrease in braiding and increase in size before the final configuration was reached in early August

• The conduits were found to be full of water during periods of higher daily discharge with water pressure exceeding atmospheric pressure

• Overall, variation in tracer return curves and dispersivity on a diurnal time scale reflect changes in the degree of braiding as lower divides are drowned between channels and discharge increases

Fountain et al., 2005

• The prevailing hypothesis explains that water flow through the body of a glacier takes place in a network of tubular conduits; however, video images from 48 boreholes drilled into Storglaciӓren, Sweden, shows that the hydrological system  is instead dominated by fractures converting water at slow speeds at all depths

• Fractures provide the main pathways for surface water to reach deep within the glacier whereas conduits only form in special circumstances

• These fractures are useful in explaining the evolution of the englacial water flow system, seasonal regeneration and in understanding the collapse of ice shelves and hydraulic connection between the surface and bed of an ice sheet

• Findings from this study expose claims to universality within englacial theory and instead provide evidence for spatial variation between and within glaciers

Benn & Evans, 2010

• The production, storage and transport of water has a profound influence on glacier behaviour as water contributes to glacial erosion, debris transport and deposition as a direct agent; water released from glaciers also present benefits and hazards for human populations through cultivating valleys for agriculture, hydroelectricity and flooding

• The contrasting permeabilities of snow and ice mean that supraglacial drainage systems on snow-covered surfaces and bare ice are very different; on snow the water readily percolates through pore spaces until it reaches the freezing point, this kind of drainage becomes more efficient over time as rills and surface channels form; for ice surfaces the meltwater cannot percolate as the ice is impermeable so much run off the surface in sheet flows or channels 

• The englacial drainage system conveys surface meltwater to the bed of glaciers, for many years models of englacial drainage were based on the theoretical model developed by Shreve in 1972 but this did not provide a realistic picture of actual drainage as they show no tendency to follow theoretical potential gradients and do not behave as predicted

• Insights into the character of englacial drainage systems have been developed through video imaging of boreholes, ice cores, glacio-spelology and surface geophysical surveys that reveal important insights to challenge assumptions of conduits being semicircular without variation, that these dominate water flow and into their seasonal, diurnal and geographical variation

• Subglacial channels may be incised into the ice or cut into the glacier bed, they develop through creep closure leading to the development of branches drainage systems that can fluctuate in response to diurnal and weather related variations in melting; these channels cannot always be assumed to be semicircular but instead have complex forms, travel in many directions and are subject to strong seasonal variation

Miles et al., 2020

• The hydrological characteristics of debris-covered glaciers are known to be fundamentally different from those of clean-ice glaciers, this influences the timing and magnitude of meltwater discharge to impact communities who rely on this for sanitation, irrigation and hydropower 

• The rugged surface of debris-covered glaciers means supraglacial systems are likely to involve channels and ponds as well as unknown pathways; englacial conduits are frequently abandoned and reactivated as water supply changes, new lines of permeability are exploited and drainage captured as well as being subject to the seasonal influence of monsoons that reorganise these systems rapidly

• This study used dye tests on debris-covered Khumbu Glacier in Nepal to infer the existence of a channelised subglacial drainage system that discharge large volumes of heavily debris-laden water during the melt season

• It was inferred that the subglacial system increased in efficiency and interconnection throughout the melt season; this channelised system was revealed but there was no evidence found for the evolution of the system from, or returning to, cavities showing greater spatial variability in subglacial drainage systems around the world

Church et al., 2020

• Between 2012 and 2019 repeated GPR surveys were carried out over an active englacial conduit network within the ablation area of temperate Rhonegletscher in Switzerland, this showed that during the summer melt seasons there are active, water-filled, sediment transporting englacial conduit networks 

• The surveys provided evidence that the conduits were up to 20m wide and formed by hydraulic fracturing 

• During the winter, the englacial conduit no longer transports water and became physically closed or very thin to produce, following the melt season this reactivated in the exact same position

• This highlights the importance of observations in testing englacial hydrology theory to allow it to capture greater spatial and temporal variability

Campbell et al., 2006

• Dye tracing was conducted in the 2004 melt season at Haut Glacier d’Arolla, Switzerland, to better understand the role of the supraglacial snowpack in mediating the delivery of meltwater produced by the snowpack surface to the rest of the glacier with implications for proglacial forms and subglacial water pressure

• Observations show the complexity of flow patterns to yield average flow rates for percolation through the snowpack of between 0.13 and 0.49 m/h with an increase in percolation rates over the course of the melt season in the efficiency of which the snowpack transmits water 

• The reason for an increase in percolation rates over the course of the melt season is attributed from the progression of the snowpack from an initial cold, impermeable surface to a warmer, better connected layer facilitating more rapid percolation by the end of the melt season and proving that the dampening effect of the snowpack varies seasonally  

• Snow permeability in this study was found to be significantly lower than previous studies, showing a need for improved research on snowpack hydrology

Willis et al., 2012

• Digital elevation models of the surface and bed of a glacier can be used to calculate the subglacial hydraulic potential and infer drainage system structures, a distributed degree day model is also used to calculate the spatial distribution of melt on the glacial surface to impact water flux beneath the glacier

• Comparisons of 78 dye tracing tests over 33 injection sites as well as measures of water discharge suggest that the temporally and spatially averaged steady state water pressure beneath the glacier were around 70% over ice burden meaning k equalled 0.7 in the hydraulic potential equation for subglacial hydrology

• This shows that the main drainage network of the eastern half of the glacier consists of hydraulically efficient systems of broad, low channels and the smaller drainage system on the west consists of hydraulically ineffective distributed systems with very broad low channels showing how the subglacial drainage system can vary within an individual glacier

Fountain & Walder, 1998

• It is important to understand water movement through glaciers to understand glacier dynamics, glacier-induced floods and predict runoff

• Firn can influence the supraglacial drainage system as when it is porous and permeable the flux of water to the glacier interior varies slowly due to the firn temporarily storing water and smoothing out variations in supply through the dampening effect; however, in the firn-free ablation zone the flux of water depends directly on the rate of surface melt or precipitation

• In the englacial drainage system the water moves from the surface to the bed through a network of steep conduits that deliver water to the bed, in the accumulation zone these are steady state features that convey water delivered via firn so are usually full of water and pressurized; in the ablation area they are only pressurised near times of daily flow or during storms 

• The subglacial drainage system can be made up of many distinct elements from a branching network of channels to a more extensive network of nonarborescent channels converting water slowly and poorly connected; the arborescent channel largely collapses during the winter but reforms in the spring as the bed disabilities cavities in the non-arborescent network 

• The volume of water stored by glaciers varies diurnally and seasonally with daily fluctuations of up to 20-30 mm; most water storage is likely to occur englacially and if this is abruptly released it can lead to catastrophic flooding

• This shows that there are great variations in the hydrology of an individual glacier between the accumulation and ablation zone

Cuffey & Paterson, 2010

• The hydrological system of glaciers can be broken down into three main categories; the supraglacial, englacial and subglacial systems that examine the movement of water on the surface of the ice, within the ice, and at the glacier bed

• Within each of these categories, there is a plethora of theories on how runoff, transportation and drainage systems are characterised; these often first come from widely accelerated theoretical analyses of hydrological processes that are then tested, critiqued and improved through the observations and the application of empirical data

• Observations are collected from the field in a number of water including drilling boreholes into ice, ice core examination, dye testing and glacio-speleology that can be combined with remote sensing methods including ground-penetrating radar (GPR) analyses

• There remains a need to continue to develop observation techniques and combine them in innovative ways to fill gaps in knowledge on glacier systems

UNESCO World Water Assessment Programme, 2025

• In March of 2025, the UN World Water Development Report found that the 2 billion people worldwide who depend on glacier meltwater for sanitation, irrigation and water security were set to face ‘severe’ consequences to their livelihoods as glaciers continue to melt as a result of anthropogenic climate change

• This impact is exacerbated as glacier melt has a profound effect on glacier hydrology by changing the volumes and timings of runoff leading to glacier behaviours changing and impacts to humans being altered

• Glacier hydrology theory is key to predicting the extent of these changes and ensuring appropriate measures are taken to reduce risks to populations that rely on glaciers to support their lifestyles

Milner et al., 2017

• Glaciers cover around 10% of Earth’s land surface but are shrinking rapidly across most parts of the world leading to cascading impacts to downstream systems as a result of changes in river hydrology and morphology bought on my glacier shrinkage 

• Understandings of glacier mass loss have improved significantly in the past decade due to advances in remove sensing and processing direct measurements, there should be increased attention to how climate driven changes in glacier volume alter the timing, magnitude and frequency of downstream discharge, sediment transport and nutrients with far reaching impacts to ecosystems, societies and human activities 

• Glacier runoff typically peaks during the summer when runoff from other sources is lower to act as a buffer against dry season stream discharge from precipitation variability; however, as glacier cover declines this annual runoff will be reduced due to a decline in glacier volume after an initial earlier peak caused by the earlier disappearance of reflective snow cover to lower surface albedo 

• These glacier runoff changes replace predictable hydrographs with more variable hydrological regimes fed mainly by unpredictable rainfall and snowmelt runoff regimes; this will lead to an increase in summer water temperature due to a decline in contribution of cold water from glacier melt, increase in air temperature and reduced heat capacity as well as a decrease in sediment load as glacial erosion reduces

• There will be a decrease in glacially supplied dissolved atmospheric organic carbon, nitrogen, phosphorus and other organic elements which may lead to a decrease in biodiversity as streams and rivers physical and chemical conditions change, this means organisms unable to adapt become extinct such as salmon and trout as a result of rising temperature; this also has a knock-on effect on the food web

• Seasonal predictable melt facilitates a range of important socioeconomic ecosystem services, disruptions to these are particularly important in semiarid and arid regions where water source contributions are limited; this can impact irrigation, hydropower, fisheries, water quality and cultural services

Chen et al., 2017

• Meltwater from glacierised catchments is one of the most important water supplies in Asia meaning the effects of climate change on glaciers and snow cover has significant consequences to water security and further uses; hydrological modelling is therefore a key research approach to water resource management in glacierised river basins in central high mountain Asia

• Hydrological models help researchers understand past and current changes and provide a way to explore the implications of management decisions and imposed changes; on a basic scale this helps support decision-making for water resource management but this relies on choosing the most suitable hydrological model for a particular watershed

• The Tienshan Mountains span several countries and sub regions in central Asia forming a political entity of multi-national and multi-ethnic forms; there are three large transboundary international rivers which may be hard to govern due to political tensions resulting in fragmented research and development of hydrological modelling particularly since the collapse of the Soviet Union

• There are a number of hydrological models applied in glacierised catchments in central Asia, these are often conceptual or distributed but continually fail to integrate processes of glacier and snowmelt runoff as well as glacial mass balance processes that are expected to influence long term declines in runoff as well as short term increased variation

• There are limitations of available hydrological models due to meteorological input uncertainty, a lack of stations due to complex terrain, a lack of scalability, lack of homogenous testing, remote sensing and reanalysis bias and a lack of inclusion of validated data on glacier melt, accumulation and mass balance

• Calibration and validation should also be improved to cover larger timescales and include a number of hydrological processes in multi-calibration 

• There is a need to include more remotely sensed data, increase use of multi-calibration, improve incorporation of glacial physical processes and overcome political tensions in the region

Pellicciotti et al., 2014

• Switzerland is one of the countries with some of the longest and best glaciological data sets for glaciers and their response to climate change; common agreement sees projected glacial runoff characterised by an initial peak followed by a long term decline due to shifts in seasonality, earlier melt onset and reduced summer runoff as runoff regimes shift from being influenced mainly by glacial melt, to variable snowmelt, to precipitation

• Predictions are difficult to compare due to differences in structure, calibration, input data, resolution and the application of future scenarios as well as key sources of uncertainty in the importance of extrapolating air temperature and the role of debris cover

• Ablation is modelled using simple degree day models, empirical models or enhanced temperature index models that all struggle to represent ice flow and simulate evolutions in glacier geometry 

• There are few studies modelling the impact of debris cover on melt despite being recognised as important in altering glacial retreat, this is especially important in the European and Swiss Alps where debris is formed from discharge from rocks or the melting of englacial debris with the impact of enhancing ablation when thinner than the critical value due to changes to albedo but reducing it when they are thicker due to insulation

• Only a few distributed models have been developed to quantify the effect of debris on ablation and runoff, these should be improved to model heat fluxes and surface characteristics in physically based models for melt of debris covered glaciers

Van Tiel et al., 2020

• Glaciers are essential for downstream water resources and hydrological modelling is key for understanding future projections of water resources in changing systems

• Major model challenges include high uncertainty in input data, mainly precipitation as a result of scarce observations leading to a risk of compensating with model errors in snow and ice accumulation; calibration is used to address this

• A large number of models use streamflow to address performance, but most now use additional data related to snow and glaciers through stepwise or multicalibration

• There is a need calibrate models through a number of variables separate from streamflow, including mass balance data, snow cover and satellite data as well as reposting uncertainty through a range of model possibilities to improve system understanding and model simulations

Hock, 2005

• Glacier melt is determined by the energy balance at the glacier-atmosphere interface, this is determined by the meteorological conditions above the glacier and the physical properties of the glacier itself; the atmosphere supplies energy for melt and thai is modified by the specific properties of snow and ice

• The degree-day model assumes an empirical relationship between melt and air temperature; data here is easy to access and extrapolate meaning it is widely used in melt computations; this simplifies complex processes of surface energy balance but matches the performance of the energy balance model, this may be due to degree day factors for snow and ice taking into account components of the energy balance with efforts to incorporate wind speed, vapour pressure or radiation to increase complexity DDFs vary in space and time, and the DDF for ice is higher than snow due to lower albedo

• The energy-balance model is a physically based approach involving the assessment of the energy flux to and from the surface including radiation, heat fluxes, precipitation and energy consumed by melt in point or distributed models; compete and distributed energy balance measures are only available over small periods and scales due to the huge volume of equipment and maintenance needed, this means computations and extrapolations are often used; this has shown that most of the energy used for melt is supplied by radiation, followed by sensible heat flux and only a small amount from latent heat

• Albedo is a key source of uncertainty for energy-balance models alongside turbulent fluxes as the average reflectivity of snow controlling the spatial and temporal distribution of melt production; summer snowfall events can impact this as well as surface and sky conditions for snow albedo; ice albedo is less studied and often treated as a constant although proven to change with dust and debris deposits

Davies, 2020

• The degree day model assumes that for each 1 degree a certain depth of snow will be melted, this takes into account the amount of energy available over the course of the year and how much melt occurs per degree in the DDF; this is a simplified model to predict glacier melt 

• The positive degree day sum is the total average temperatures over 0 degrees in a given time period, and the DDF is the amount of melt that occurs per positive degree day, these can be measured on a glacier using ablation stakes

• DDFs are subject to significant local-scale variability on the same glacier, they are also different for snow and ice due to differing albedos

Zhang et al., 2006

• Spatial variations in DDFs greatly affect the accuracy of snow and ice melt modelling, on a single glacier DDFs are subject to significant small-scale variation due to unique climatic conditions and heat subjects for certain locations

• Depending on the spatial variation of DDFs and meteorological data degree-day models can be formed to reconstruct glacier mass balance incorporating spatial heterogeneity 

• With different elevations, solar radiation and surface albedos DDFs are found to vary considerably in western China, this is likely also impacted by topography

Braithwaite, 1995

• Ice ablation is related to air temperature by the positive degree-day factor; the DDFs vary with summer mean temperature, albedo and turbulence 

• Variations in DDFs due to changes in albedo and turbulence can explain high DDFs at low temperatures; remaining uncertainties can be reduced by better knowledge of ablation variation in remote parts of glaciers, this should incorporate better knowledge of albedo and turbulence conditions

Hock, 2003

• Degree day models rest on the relationship between snow and ice melt and air temperature for positive degree days, this uses readily available data in simplistic models; however, these have shortcomings as over long periods accuracy decreases as well as spatial variability being impacted by the topographic effects of shading, slope and aspect angled crucial in mountain areas

• Degree day models have been the most common for melt modelling due to the availability of air temperature data, easy interpolation, good model performance and computational simplicity 

• There is large variability from site to site in DDFs that come from direct measurements using snow lymister outflow, ablation stakes or computations, this variation can be attributed to differences in the relative importance of energy components for melt such as sensitive heat flux and high radiation areas

• DDFs are expected to increase with increasing elevation, increasing solar radiation input and decreasing albedo as well as varying seasonally due to changes in solar radiation and changes in albedo; due to this spatial and temporal variability DDFs must be adjusted to each application in degree-day modelling

Arnold, 2005

• The amount of glacier melt depends on the surface energy balance, which depends on an interplay between the surface conditions of the glacier, external meteorological conditions and the topography of the glacier surface and surrounding areas; this was investigated at Haut Glacier D’Arolla, Switzerland

• A key condition here is the roughness of the surface, as ice surfaces tend to be rougher and have a lower albedo than snow surfaces; therefore melting more quickly under a given set of meteorological conditions

• During the melt season the snow line retreats up the glacier to exposure rougher, lower albedo ice which melts faster than remaining snow cover leading to nonlinear mass-balance profiles

• Energy availability also varies over the surface of the glacier with solar radiation providing the largest part of energy for melting, at higher elevations potential solar radiation is higher due to a lack of shading from surrounding topography

• Spatial variations in potential solar radiation and albedo exert a profound control over the form of mass balance of a glacier, this will strongly affect the response of glaciers to possible future climate change

Braithwaite & Zhang, 2000

• A degree day model was used to assess the sensitivity of mass balance of five Swiss glaciers to temperature changes, this was tuned with precipitation to show that the largest mass balance changes occurred at the snout with a progressive increase with altitude

• Model accumulation fell by 5-8% with a temperature rise of 1 degree reflecting the reduced proportion of snowfall at higher temperatures; precipitation increases may be able to partially offset the effect of higher temperatures but decreased precipitation would reinforce the effects of increased temperature

• There is a strong relation between sensitivity and altitude suggesting that climate change experiments may be better performed for altitude bands rather than whole glaciers

Hugonnet et al., 2021

• Glaciers distinct from the Greenland and Antarctic ice sheets are shrinking rapidly and altering regional hydrology, raising global sea level and elevating natural hazards 

• During 2000 to 2019 glaciers lost a mass of 267 Gt per year, equivalent to 21% of observed sea level rise; there is a mass loss acceleration of 48 Gt per year per decade explaining 6 to 19% of observed acceleration of sea-level rise

• Observations and predictions of glacier changes show the critical need to design adaptive policies for local and regional scale management of water resources and cryospheric risks as well as global scale mitigation of sea level rise; this is critical to avoid water scarcity, sociopolitical instability and impacts to ecosystems 

• This study used large scale satellite and elevation datasets on a global scale

Hock, 2017

• Hourly melt and discharge of Storeglaciaren, Sweden was computed using a temperature-index model on a 30m resolution grid; this improved on the classical degree-day model that was useful for seasonal patterns of discharge but did not capture pronounced temporal and spatial variability 

• Degree data models have been widely used due to air temperature being the most readily available data however it fails to capture high temporal resolution on less than a daily scale as well as spatial variability resultant from topographic shading, aspect and slope angles; such drawbacks can be improved upon by including clear-sky direct solar radiation

• In the temperature-index model melt is computed from the classical degree-day model, potential clear-sky direct solar radiation and global radiation strongly affected by atmospheric conditions, local topography and clouding

• The results showed a pronounced spatial heterogeneity as melt rates in south-facing slopes exceeded north-facing slopes due to the effects of topography; this also led to strong diurnal cycles in melt

• This hybrid model is a useful alternative to energy balance models when heat flux data is restricted as well as improving on the classical degree day model by capturing melt rate variation in space and time whilst solely based on air temperature measures

Benn & Evans, 2010

• Mass balance refers to the change in mass for all or part of a glacier over a specific time period, the net annual mass balance is the sum of accumulation and ablation over the calendar of balance year; this can be for a particular location on the glacier as specific mass balance

• Direct measurements occur in pits excavated from snowpacks or cores or ablation stakes; geodetic measures from areal photographs or satellite data are also increasingly used to allow for better areal coverage over complex terrain as well as gravimetric methods using GRACE identical orbiting space craft to detect small changes in the Earth’s gravitation field as a result of changes in mass balance with great sensitivity

• The amount of snow and ice stored in glaciers undergoes systemic changes throughout the year following cycles of gain and loss depending on seasonal distinctions between accumulation and ablation to form annual mass balance cycles highly correlated to seasonal shifts 

• Climatic changes to precipitation or other energy balance components will impact the mass balance of the glacier, shifting the equilibrium line and leading to a positive or negative net mass balance; different glaciers have different levels of climate sensitivity

He et al., 2021

• Quantifying contributions of glacier runoff components to stream flow is key to understanding the dynamics of water resources under changing climates in glacierised basins; this is done using tracer-based end-member mixing methods or hydrological modelling

• Hydrological models integrate the dynamics of water storage, runoff, snowpack thickness, rainfall etc, to physically represent the dynamics of water storage in the snowpack and glacier body corresponding to runoff generation; uncertainties can stem from a large number of parameters and a lack of agreement on processes included

• End member-mixing models use water tracers such as stable isotopes, electrical conductivity and hydrochemistry tracers to estimate contributions to runoff and are typically used in smaller basins on seasonal rather than annual scales; uncertainties here are based on spatio-temporal variability of tracer signatures, these uncertainties tend to be larger than hydrological modelling

• Using end member-mixing models over 321 globally glacierized basins it was found that on an annual timescale, the most important factor including basin characteristics where glacierised area ratio and mean basin elevation for ice melt contribution

Huss & Hock, 2018

• Worldwide glacier retreat and associated future runoff changes raise major concerns over the sustainability of global water resources

• In a global analysis it was found that roughly half of 56 large-scale glacierised basins saw runoff continue to rise until a maximum peak water is reached beyond which this runoff declines, in the remaining basins this tipping point has already been past; it is in basins with large glaciers and high ice cover fractions where peak water occurs later

• This pattern occurs as mountain glaciers begin to retreat due to climate change and release water from long term storage leading to an increase in runoff until a maximum is reached, after this tipping point runoff decreases as reduced glacier areas cannot support rising meltwater volumes

• It has been found that peak runoff has been reached in 45% of basins with annual runoff now expected to decline; however, runoff is still increasing in 22% of basins due to larger glacier areas and high fractions of ice cover

• The largest reductions in runoff are seen in central Asia and the Andes, but downstream hydrological effects of continued glacier recession are substantial for most of the world with magnitude varying among basins and throughout the melt season

• Runoff reductions are of greatest concern with combined with high population densities in remote downstream communities

Bolch et al., 2022

• The river Aksu in the Tarim region of northwestern China depends solely on water sources from glacierised mountains as a result of an arid climate; however, these glaciers have had a negative mass since 1975, as a result discharge has been increasing over the last decades due to glacier melt, but is modelled to reach a tipping point between 2040 and 2060 before declining thereafter 

• The availability of water is key in this arid area for the sustainable development of cities, agricultural land and ecosystems such as the Tugai forests that regulate climate, prevent desert expansion, protect from sandstorms and are a key source of firewood; there is a strong dependence and competition for this water

• Water demand in the region has increased due to irrigation and related industry expansion as well as increase in growing water dependent crops, this cannot be met with future declines in runoff modelled using hydrological models WASA and SWIM-G meaning there is likely to be a substantial water shortage in the future

Benn & Evans, 2010

• Glaciers are not uniformly cold as they can differ in temperature between and within individual glaciers; temperature is calculated against the local pressure melt point as the temperature at which ice melts decreases with pressure

• Energy exchange between the glacier surface and atmosphere is a key control as energy flux can raise of reduce the temperature of near-surface snow or ice, refreezing can also release latent heat to warm ice even when air temperature is below freezing; climatic changes alter energy exchanges with the atmosphere

• Geothermal heat flux is the upward transfer of heat resulting from the decay of radioactive isotopes within the Earth, this depends on the tectonic setting and raises the temperature of basal ice 

• Frictional heat can be generated englacially by internal deformation, subglacially by basal sliding or by the deformation of subglacial sediments; heat can also be conducted through snow and ice with efficiency determined by thermal conductivity which is lower for snow due to the presence of air pockets

• Temperature distributions are also influenced by glacier flow transporting ice from high to low elevations meaning the temperature of ice at one location depends on initial temperature at formation, thermal evolution and the dissipation of heat by conduction to colder areas

• Temperate glaciers are everywhere at the melting point apart from a surface layer subject to seasonal cycles, they tend to be found in temperate maritime locations with high precipitation and summer melting; cold glaciers are everywhere below the melting point and frozen to their beds found in cold, arid environments 

• Polythermal glaciers are the most geographically widespread and are composed from cold and warm ice, they have a range of thermal structures depending on the balance of surface and subsurface warming and are found in polar maritime climates

Irvine-Fynn et al., 2011

• The manner in which meltwater drains through a glacier is critical to dynamics, runoff and water quality, yet much knowledge is based on temperate glaciers meaning there is a need to examine nontemperate hydrological processes and reject a unified model of drainage architecture; this is especially key to model ice mass responses to climate change

• Temperate glacier hydrology is defined by a permeable surface, crevasses and moulins in the englacial system of conduits and a seasonally varying subglacial drainage system; however, this cannot be as easily applied to nontemperate glaciers

• Due to the cold ice content of nontemperate/polythermal glaciers there is reduced ice deformation and fracturing resulting in low crevasse density; instead a weathering crust can form to storage water in a near surface thermal layer confined aquifer to act as a dampener, well developed surface drainage systems also form through incision on the ices surface 

• These supraglacial streams can develop into englacial conduits through cut and closure to form high roofed englacial channels mirroring the meandering form of supraglacial forms, this leads to the formation of classical englacial moulins which appear anomalous on glaciers devoid of crevasses; these can also form due to hydrofracture propagation and channel interception at structural weaknesses such as debris bands

• At the subglacial level, thin films of water can exist, yet there is little known about this showing a need to research nontemperate glaciers further through field research, data collection and modelling conditions and their responses to climate change

Miles et al., 2018

• Runoff from high elevation debris covered glaciers is a critical water supply for many in the Himalayan regions, knowledge of thermal regime is key for predicting responses to mass balance and drainage pathway changes that influence discharge in response to future climate change; yet a lack of knowledge on internal ice temperatures makes projections uncertain

• 13 boreholes were drilled into the ablation area of Khumbu Glacier with multiple thermistor strings at different depths inserted into each; this found that the glacier is polythermal with cold ice in the upper ablation area and temperate ice in the lower part beneath a seasonally influenced upper layer of around 10m

• The lower ablation area was found to contain around 56% temperate ice with a minimum temperature of -3.3 degrees, 2 degrees warmer than annual air temperature showing vulnerability to even minor atmospheric warming and possible increase in supraglacial pond formation and englacial/subglacial drainage systems

Blatter, 1987

• From 1974 to 1981, 32 boreholes were drilled on White Glacier and vertical ice temperature profiles measured

• This saw an extensive layer of temperature ice discovered close the the bedrock in the lowest part of the glacier tongue, this was not in a steady state but the temperature minimum was explained by general climatic warming since 1880 during the Little Ice Age 

• The surface layer temperatures show relations to climatic conditions and balance zones, and the thermal regime is determined by internal heat flux of diffusion and advection

Copland & Sharp, 2017

• Ground-based radio-echo sounding involves transmitting a radio wave into the glacier and then measuring its frequency when it returns, this indicates different conditions below the glacier based on residual bed reflection power that can be interpreted in terms of thermal and hydrological conditions at the base of polythermal glaciers

• Radio-echo sounding can show boundaries between warm and cold ice, the presence of ice at the PMP and cold ice on the bed; it is particularly good at inferring hydrological conditions at the bed due to the contrast between ice and water being higher than between ice and other subglacial materials

• At John Evans Glacier, a high-Arctic polythermal glacier in Canada this identified a marked contrast between warm based areas of the ablation zone and the cold based accumulation zone that were attributed to differential englacial ice temperatures and variations in subglacial hydrology that could be reconstructed

• Data collection by radio-echo sounding is rapid and inexpensive, it is able to provide details information about subglacial properties over large areas and works best for polythermal glaciers where there is marked variation in hydrological and thermal regimes

Bjornsson et al., 1996

• Radio-echo soundings are an effective tool for mapping thermal regimes of polythermal glaciers as electromagnetic waves penetrate the ice and reflection frequency shows which layers are at the melting point and contain water; this has been used to investigate four polythermal glaciers in Svalbard including Midre Lovenbreen and Austre Broggerbreen 

• Information on the thermal regime of Svalbard’s polythermal glaciers has come from hydrological characteristics, temperature measurements in boreholes and radio-echo sounding which showed strong internal reflections interpreted as showing the interface between cold and temperate ice 

• This showed that Midre Lovenbreen was frozen to the bed at the snout and along all mountain slopes, but was warm based beneath the central part

• On Austre Broggerbreen a temperate basal layer was not detected using radio-echo sounding, but the basal ice was observed to be at the melting point in two boreholes

Gusmeroli et al., 2012

• Ice-penetrating radar surveys from 1989, 2001 and 2009 show ongoing complex thinning of the cold layer of Storglaciaren having lost ⅓ of the cold surface layer volume in 20 years as a result of Arctic climatic warming 

• The effect of climate change on glaciers is a key concern as glacier-runoff directly affects sea level and water resources as well as the thermal structure of glaciers, this can lead to rising englacial temperatures reducing friction on beds to increase ice avalanching 

• Storglaciaren is a largely temperate glacier apart from a cold surface layer overlying the ablation area, yet this is thinning with great spatial variability in the ablation area as a result of reduced accumulation and increased summer melting, this impacts the thermal regime more broadly 

• Climate is the dominant control on cold-layer thickness allowing the cold-temperate transition surface (CTS) to migrate upwards and the temperate core to expand at the same time as mass balance is increasingly negative, this can change the internal direction of the ice with impacts to how the glacier moves and reacts to seasonal melt

Hodgkins et al., 1999

• Scott Tunerbreen in Svalbard has lost significant mass since surging in 1936, this mass loss has been accompanied by a transition from polythermal to frozen thermal conditions at the bed meaning the glacier is unable to surface again showing a complete transition in glacier geometry, thermal structure and dynamics of the glacier

• There has been substantial retreat and thinning of the glacier between 1936 and 1993 due to the period following the surface combined with rising temperature since the termination of the Little Ice Age, this has led to a lack of insulation from a thick layer of ice allowing cold temperatures to permeate to the base and freeze the ice to the bedrock

• The change in thermal regime from polythermal to frozen meanings surging conditions are now impossible

Blatter & Hutter, 1991

• For a polythermal glacier, a mathematical model is used to calculate temperature in the cold part of the glacier and the position of the cold-temperate transition surface (CTS) where the ice changes phase and temperature 

• The thermodynamic model is driven by the results of a geometry and flow field separate dynamic model to represent the polythermal structure of the glacier within a range of prescribed conditions

• When this was applied to Laika Glacier in Canada, the model yielded a wholly cold glacier with a cold base contrary to observations of a polythermal current state; this showed that the glacier was not in equilibrium with present climate 

• When run with conditions of the Little Ice Age, a more realistic polythermal structure was formed to prove that this thermal regime does not result from present conditions, but from the climatic conditions and geometry of the Little Ice Age that remain stable under climatic conditions that are not quite sufficient to produce it

Gulley et al., 2009

• Shreve’s theoretical model formed in 1972 key provided key influence on the character of englacial drainage systems based on three assumptions that systems are (1) at steady state, (2) follow the steepest hydraulic gradient and (3) the pressure head equals the pressure of the surrounding ice minus the melting of the walls; this has been widely adopted despite no evidence that this is a realistic picture of actual drainage systems

• Speleological techniques including borehole measures, direct observation and geo-physical surveys in 27 distinct englacial environments showed that Shreve-type drainage systems do not exist and englacial conduits are able to penetrate through thick ice to recharge the bed through three key mechanisms

• Cut and closure sees englacial passages formed by the incision of supraglacial stream canyons and closure by ice creep in conditions where incision occurs at a faster rate than ablation; this forms high, meandering conduits

• Englacial water can exploit permeable structures such as joints, bedding planes and discontinuities; in debris-filled crevasse traces water can flow in and engage the passage to form a conduit and surface fractures can become hydraulically connected yet neither have been shown to connect to the bed

• The presence of water in crevasses can also lead to hydrologically assisted fracture propagation as this alter the force balance by countering the overburden and allowing for deep crevasses to reach the bed

Gordon et al., 2001

• The study of glacier hydrology relies on measurements made in boreholes to construct the behaviour of subglacial drainage systems; yet this often ignores how boreholes remain open to the atmosphere meaning the interpretation of data may be complicated by supraglacial and englacial water flows to and from boreholes

• Experiments at Haut Glacier d’Arolla, Switzerland showed that the boreholes have their own drainage characteristics and can be classed as connected to different parts of the drainage system, unconnected and unconnected blind

• This complicates interpretations of water-levels in boreholes, meaning they must be supplied with additional measurements to be viewed as accurate and can provide new systems of englacial drainage

Fischer et al., 2005

• Digital elevation models of the bed and surface of Unteraargletscher, Switzerland can reconstruct patterns of basal water drainage from 1927-97 when the glacier was thinning and receding; these changes to glacier geometry and subglacial drainage influence glacier dynamics, mass balance and runoff

• As the glacier thinned over time, moraines were able to become more pronounced and increase in height due to a lower melt rate resulting from increased insulation from debris cover

• Under these moraines there were areas of increased hydraulic pressure combined with ice overburden pressure decreasing as the surface thinned; this leads to hydraulic barriers but also increased channelisation at the bed

• Here, the k value was initially assumed to be 1, but it was shown that a k value of 0.5 was more appropriate showing a greater influence of bed topography on the hydraulic potential

Willis et al., 2009

• GPR and dye tracing was used at Brewster Glacier to derive maps of subglacial hydraulic potential and drainage systems structure; it is important to compare water pressures and drainage systems to show how drainage may be constrained by differing subglacial water pressures 

• The data shows that the glacier is underlain by a channelised but hydraulically inefficient drainage system in early summer as water pressures are high and too close to the ice overburden; by mid-summer water pressures lower to half the ice overburden pressure allowing for more hydraulically efficient drainage

• There is a more efficient drainage pathway near to the snout, but streams entering further away from the major drainage pathways are routed more slowly as they must first flow through a distributed drainage system

Hubbard et al., 1995

• Later-summer subglacial water pressures are measured using boreholes in the ablation area of Haut Glacier d’Arolla; this revealed a main subglacial channel along a longitudinal variable pressure axis surrounded by distributed systems adjacent to it

• Water pressure variations recorded in boreholes close to the VPA show strong diurnal cycles as a result of efficient channelised drainage of day time melt; the surrounding distributed boreholes consist of less efficient channels and therefore maintain high water pressures with little variation; the efficiency of the distributed system increases closer to the channel 

• This is useful in understanding the co-existence and interaction between two subglacial drainage systems at a single glacier

Irvine-Fynn et al., 2005

• Over the course of the 2004 and 2005 meltseason a number of dye injections were conducted using entry points and moulins found in crevasse zones close to the accumulation zone; surface streams can also be examined using dye traces

• Dye emergence can be detected by continuous fluorometry using fluorometres

• Results from tracing experiments can be used to calculate the internal transit velocities of water within the hydrological system of the glacier

• At Midre Lovenbreen, Svalbard this revealed efficient englacial systems, potentially linked by a fracture flow network which recharged the subglacial system

Romeo & Piccini, 2022

• Gorner Glacier in southern Switzerland is one of the most studied glacier in the world regarding englacial caves as Italian speleologists first researched it by dissending into moulins in 1985 and it has since been surveyed annually 

• Several moulins up to 135 metres deep and numerous contact cavities are surveyed annually to observe the development of englacial drainage structures and the seasonal evolution of active shafts

• The impact of global warming has been observed in the internal drainage network as moulins move downstream as a result of changing flow conditions and thinning leading to a reduction in the maximum explorable depth inside moulins

Barthold et al., 2011

• End member mixing analysis is commonly applied to identify and quantify the dominant runoff producing sources of water by applying tracers to determine the dimensionality of the hydrologic system 

• Many EMMA studies have used 2-6 tracers with main elements being calcium or chlorine, isotopes or alkalinity; few studies use larger tracer sets of use minor trace elements such as lithium 

• Conceptual models are highly sensitive to the tracer set size and competition; major elements are also not always the most useful tracers and larger tracer sets have an enhanced capacity to avoid uncertainties on catchment functioning

Bindschalder et al., 2017

• Recent measurements of temperature and velocity from Variegated Glacier, Alaska show that the glacier is sliding over much of its length which requires that the base of the glacier is temperate 

• Temperature measures using borehole thermometry show that Variegated Glacier is temperate throughout and this allows for surging behaviour alongside water introduction to the glacier

Rahman et al., 2015

• In the Swiss Alps end member mixing was used at Mutt watershed at the Rhone basin to quantify contributions to runoff as biodiversity in the area is dependent on meltwater 

• This used electrical conductivity, water temperature, sulfate tracers and chlorine tracers in order to distinguish between glacial melt water from Mutt glacier, surface runoff and groundwater 

• This showed that glacial melt has a strong diurnal variation which influences the magnitude and timing of peak flow, these fluctuations are correlated with environmental variables including temperature and the slope of the watershed

• There were uncertainties here in reliably characterising end member properties based on inaccurate stream chemistry measures

Lai & Anders, 2021

• Climate has been viewed as a primary control on the rates and patterns of glacial erosion; however, the basal thermal regime may also be a first-order control 

• Polythermal glaciers contain both cold-based portions that protect bedrock from erosion and warm-based portions that actively erode bedrock

• Cold temperature lead to limited glacial erosion at high elevations due to cold-based conditions, increasing precipitation can overcome this impact by accumulating thick ice and lowering the melting point at the base

• High precipitation rates therefore tend to cause warm-based conditions at high elevations, resulting in intensive erosion at the bed